Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November, 2015 Denver Whittington Stuart Mufson Bruce Howard Indiana University
Outline ➢ Goal: Measure the time structure of the scintillation signal from liquid argon after excitation by cosmic-ray muons DUNE DocDB# 696 – to be submitted to JINST on Nov. 6 ➢ ➢ Experiment TallBo ➢ Light guide designs ➢ Silicon photomultipliers ➢ ➢ Scintillation structure analysis ➢ Physical model of signal ➢ Comparison of Models ➢ Results D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 2
TallBo ➢ TallBo at Fermilab (PAB) 84” LAr dewar ➢ Data collected Nov./Dec. 2014 ➢ ➢ Ultra-high purity liquid argon Vacuum to remove residual atmosphere ➢ Condenser to maintain closed system ➢ Active N 2 , O 2 , and H 2 O monitoring ➢ ➢ O 2 ~40 ppb (negligible) ➢ N 2 < 200 ppb (negligible) ➢ H 2 O ~8ppb (negligible) ➢ Multiple light guide designs Dip-coated acrylic bars ➢ Cast acrylic and polystyrene bars ➢ ➢ Hodoscope (cosmic ray) trigger 2 8x8 Arrays of PMTs + BaF crystals ➢ ➢ CREST cosmic-ray balloon experiment 2 scintillator paddle planes ➢ ➢ Allows shower rejection, reconstruction of single tracks D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 3
Light Guides ➢ Large active area UV-collecting light guides Acrylic or polystyrene imbued with wavelength-shifting compound ➢ ➢ 20 inch prototypes tested in this experiment 128 nm VUV scintillation signal converted to visible by WLS ➢ 430 nm visible light transported via total internal reflection to end ➢ ➢ Four light guides analyzed D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 4
Silicon Photomultipliers ➢ Biased at 24.5 V (low noise, high gain) ➢ Excellent single-pixel resolution → ➢ Characteristics measured in LN2 for each of the 12 SiPMs 6 Gain ~ 3.5 x 10 ➢ Noise ~ 9 Hz ➢ Cross-Talk ~ 20% ➢ Signal shape (rise & recovery times) ➢ all waveforms average waveform D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 5
Signals from Cosmic-Rays ➢ Example waveform from a hodoscope-selected track Prompt multi-photon pulse from ➢ early light (~20 pe here) Lots of few- or single-pe pulses ➢ from late light All convolved with the SiPM's ➢ response shape all waveforms average waveform ➢ Superposition of all cosmic-ray waveforms collected by one SiPM, with average cosmic-ray response inset D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 6
Analysis ➢ Average signal is convolution of illumination with SiPM response average time-dependent average cosmic ray waveform signal of scintillation photons measured by SiPM k “illumination function” (inset from slide 6) (inset from slide 5) average single-pe SiPM response function ➢ Use Gold deconvolution algorithm (in ROOT TSpectrum) to recover the average illumination function I ( t ) Average time sequence of ➢ scintillation photons incident on the light guide. Fast, sharp pulse from early light ➢ Long-lived tail from late light ➢ persistent for several μs Deviation from exponential ➢ fall-off at late times D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 7
Analysis – Phenomenological Model ➢ Fundamental signal expected to be an exponential probability distribution convolved with a Gaussian ➢ “Exponentially-modified Gaussian” (EMG) function ➢ Gaussian-like rise with exponential tail ➢ Multi-Component Fit ➢ Two components insufficient ➢ Best fit with found using four EMG components ➢ Early-light component ➢ Intermediate component ➢ Frequently reported ➢ Late-light component ➢ Fourth component ➢ Describes behavior at > 6 μs D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 8
Analysis – Phenomenological Model ➢ Features of note Late-light lifetime = 1.52 microseconds ➢ ➢ Compatible with other measurements Early-light fraction ≈ 25% ➢ ➢ Compatible with other measurements using beta and gamma sources Longer early light lifetime measured ➢ by polystrene light guide ➢ Likely due to additional fast scintillation from polystrene (reported elsewhere) ➢ Don't have sensitivity to resolve this substructure here Fourth component not reported before ➢ ➢ Present in acrylic light guides only D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 9
Analysis – Physical Model ➢ The intermediate and fourth components appear instrumental Likely associated with delayed emission from the wavelength shifter ➢ ➢ Data refit using a physical model description for the illumination Two-component LAr emission (singlet and triplet) ➢ ➢ Exponential probability distribution functions Three-component WLS response (1 ns, ~130 ns, and ~6.6 μs) ➢ ➢ Exponential probability distribution functions LAr emission convolved with WLS response ➢ ➢ All convolved with a Gaussian function ➢ Result is again a sum of EMG functions, reparameterized to separate WLS response from LAr scintillation A S and A T represent the true liquid argon scintillation components ➢ D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 10
Analysis – Physical Model ➢ Same quality fit, amplitudes easier to interpret Green: Emission from singlet Ar 2 * eximers in the liquid argon, ➢ converted to visible by TPB. Tail of delayed emission from WLS (~30%) clearly visible. Magenta: Emission from triplet Ar 2 * eximers in the liquid argon, ➢ converted to visible by TPB. D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 11
Analysis – Physical Model ➢ Agreement in results Singlet and triplet lifetimes agree with early- and late-light ➢ components from phenomenological fits WLS delayed emission lifetimes match intermediate and fourth ➢ components from phenomenological fits About 30% of the 128-nm scintillation signal is converted to visible by ➢ the WLS through delayed emission mechanisms ➢ Similar delayed emission recently reported in Phys. Rev. C (E. Segretto) ➢ Agrees with “early light” as the 70% of singlet light converted promptly D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 12
Prompt Fraction ➢ Additional cross-check: Calculate “prompt fraction” Fraction of signal detected within the first 40-120 ns (varies by detector) ➢ This fraction includes all ➢ early light and some fraction of the intermediate and late light. Reported values for electron sources all measure ~0.3. ➢ This study sums the first 20 SSP samples (133 ns) for comparison ➢ ➢ t * = 130 ns, t f = 10 μs ➢ Same result of F prompt ≈ 0.3 D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 13
Comparison of Models ➢ Triplet state Ar 2 * eximer lifetime measured as 1.52 μs ➢ Physical model indicates that ~30% of scintillation light is converted by WLS to visible through delayed emission ➢ Calculation of “prompt fraction” agrees with results for electrons from various dark matter and double-beta-decay experiments D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 14
Summary Recovered time-dependent ➢ structure of scintillation signal detected by DUNE light guides with SiPMs by deconvolving the average SiPM single-pe response from the average cosmic-ray signal. Phenomenological model ➢ Physical Model ➢ Measured scintillation ➢ parameters associated with cosmic-ray muons in LAr τ T = 1.52 μs ➢ Early light fraction ~25% ➢ Delayed emission from WLS ➢ ➢ ~30% effect Singlet LAr fraction ~36% ➢ Prompt signal compatible ➢ with various other electron signal measurements D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 15
Backup D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 16
Bonus: Scintillation Signal from Xenon-Doped Liquid Argon ➢ Injected xenon into the liquid argon ➢ GXe mixed with GAr, heated, and injected into the liquid at ~150 psi ➢ Increments of 20 ppm (by volume) ➢ Time structure determined using same deconvolution procedure Time-dependent structure of the LAr+Xe signal Cumulative scintillation signal from LAr+Xe (area normalized) ➢ 1.52 μs tail replaced by broad signal at ~200 ns (20 ppmv) ➢ Broad signal becomes more prompt as concentration increases ➢ Further analysis to be done ➢ Prompt signal possibly diminished ➢ Hodoscope-triggered data hints at ~50% more light from Xe-doped LAr D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 17
Excitation of Liquid Argon ➢ Charged particles create diatomic Ar-Ar eximers (Ar 2 *) e- Ar Ar 2 + 128 nm Recombination 35% 65% 5 ns Ar Ar Ar 2 * Ar+ (singlet) e- 1.5 μs μ- 50% Ar 2 * Ar* 50% (triplet) Ar Self-Trapped Exciton ➢ Result is a prompt singlet signal and a long-lived triplet signal Ratio depends on ionization properties of incident particle ➢ Intermediate signal also reported but of unknown origin ➢ D. Whittington - Scintillation Light from Cosmic-Ray Muons in Liquid Argon 5 November 2015 18
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